The present invention relates to organic light-emitting diodes with an improved contrast ratio.
An organic light-emitting diode (LED), also referred to as an organic electroluminescent (EL) device, is constructed in a normal configuration on a transparent substrate through which the light emitted by the device is viewed, and the device structure typically includes, in sequence, the transparent substrate, a transparent conductive hole-injecting electrode (also referred to as the anode), an organic hole-transporting layer, an organic light-emitting layer, an organic electron-transporting layer and an electron-injecting electrode (also referred to as the cathode) consisting of a metal having a low work function. Electron-hole recombination at or near a junction between the organic hole-transporting layer and the organic light-emitting layer results in light emission when the hole-injecting electrode is biased at a sufficient positive electrical potential with respect to the electron-injecting electrode. The highly reflective metal electrode helps to improve brightness of emission in that the electron-injecting electrode provides a surface from which internally generated light from the light-emitting layer is reflected and directed toward the transparent substrate. However, such a metallic electron-injecting electrode also reflects ambient light entering the device structure through the transparent substrate and the transparent hole-injecting electrode, thereby degrading the visually perceived contrast of the emitted light, as viewed by an observer. In numerous practical applications it is quite important that an organic light-emitting device can be easily viewed under ambient lighting conditions ranging from total darkness to full sunlight so that a sufficient reduction is required in reflection of ambient light from the mirror-like surface of the metal electron-injecting electrode. The legibility of displays under ambient lighting conditions can be quantified by defining a contrast ratio (CR):
CR=(Lon+RLLamb)/(Loff+RLLamb)
where Lon and Loff are the luminances of the on and off pixels, respectively, Lamb is the ambient illuminance, and RL is the luminous reflectance of the display. As can be seen from this expression for the contrast ratio, as RL becomes very small, CR becomes very large. Thus, even if an approach for minimizing RL results in Lon being somewhat smaller than its nominal (without any contrast reduction implementation) value, overall, CR will still be enhanced.
A well known approach for reducing glare attributed to ambient lighting is to use polarizers, particularly circular polarizers, which may be bonded to an outside surface of the transparent substrate. However, the use of polarizers adds significant cost and a polarizer bonded to a substrate is not a part of the integral layer structure of a light-emitting device.
In the construction of some inorganic light-emitting devices, one approach to enhance sunlight readability and reduction of glare has been to incorporate in such an inorganic device a light-absorbing layer and a dielectric spacer layer between the inorganic phosphor layer and the counter electrode layer. The thickness of the dielectric spacer layer is optimized to create destructive optical interference of the ambient light, thereby reducing ambient light reflection. This approach has produced inorganic light-emitting displays having 3.4% spectral reflectance [Dobrowolski et al., Appl. Optics 31, 5988 (1992)]. This approach is also discussed in U.S. Pat. No. 5,049,780 by Dobrowolski et al.
In the above inorganic structure it was stressed that in order to get very low photopic reflectance values, it was necessary to have the light-absorbing layer and the dielectric spacer layer interposed between the light-emitting layer and the reflective cathode. For this structure to be useful in organic EL devices, both the light-absorbing and dielectric-spacer layers need to be conductive, while the light-absorbing layer also must provide a work function less then 4.0 eV so as to enable adequate electron injection into the electron-transport layer. Additionally, the materials must be formable by deposition techniques which are compatible with organic EL device fabrication so as to minimize deleterious effects such as, for example, radiation damage or undesirable chemical or physical interactions between the reflection-reducing layers and the organic layers.
Thus, the requirements and specifications imposed on ambient light reflection-reducing layers useful in organic light-emitting devices are substantially different from and more stringent than the requirements for such reflection-reducing layers for an inorganic light-emitting device.
It is an object of the present invention to provide an organic light-emitting device having higher contrast ratios by reducing the ambient light reflection from the cathode.
This object is achieved by an organic light-emitting diode, comprising:
a) a transparent substrate;
b) a transparent anode layer disposed over the substrate;
c) a hole-transport layer disposed over the anode layer;
d) a light-emitting layer disposed over the hole-transport layer;
e) an electron-transport layer disposed over the light-emitting layer;
f) a thin cathode layer disposed over the electron-transport layer and having a thickness selected so that light can pass through such cathode;
g) a light-absorbing layer disposed over the cathode layer;
h) a dielectric spacer layer disposed over the light-absorbing layer; and
i) a conductive layer disposed over the spacer layer and electrically connected to the cathode so that when a voltage is applied between the transparent anode and the cathode, the light-emitting layer produces light which passes directly through the hole-transport layer and the transparent anode and substrate.
It is an advantage of the present invention to make use of an organic light-emitting structure for producing light that has large contrast ratios. It has been found that by having the transparent cathode directly over the electron-transport layer and providing a light-absorbing layer and a dielectric spacer layer before applying a conductive layer, that a highly efficient light-emitting diode structure can be produced. This structure has high contrast ratios so that it can be effectively used under varied ambient lighting conditions.